The present invention relates to a method of forming a fine pattern for various types of solid state devices. This invention also relates to a projection exposure apparatus and a projection exposure mask used for the fine pattern formation, a method of fabricating the mask and a method of layout designing of the mask pattern. This invention further relates to an optical lens used for all optical apparatuses and an optical filter installed in the optical lens.
In order to improve the degree of integration and the operation speed of solid state devices such as LSI, circuit patterns have been miniaturized more and more. At present, a reduction projection exposure method superior in mass productivity and resolution capability is widely used for forming such circuit patterns. The resolution limit of this method is proportional to the exposure wavelength and inversely proportional to the numerical aperture (NA) of the projection lens. The depth of focus, on the other hand, is proportional to the exposure wavelength and inversely proportional to the square of NA. As a result, with the improvement in the resolution limit (increase in NA and shortening of wavelength), the depth of focus is greatly reduced.
Conventionally, there has been suggested a phase-shifting method for reversing the phase of light transmitted through an adjacent aperture on the mask as a method for remarkably improving the resolution of the projection exposure remarkably. Also, a FLEX (Focus Latitude Enhancement Exposure) method for effecting exposure by the use of images of the same mask pattern formed at a plurality of positions along the light axis has been suggested as a method for remarkably improving the depth of focus in the conventional projection exposure method. The phase-shifting method is discussed in Levenson, et al, IEEE Trans. Electron Devices, Vol. ED-29, pp. 1828-1836 (1982), and the FLEX method in Fukuda, et al, IEEE Electron Device Letters, Vol. EDL-8, pp. 179-180 (1987), for example.
A method of changing the imaging characteristics by changing the distribution of amplitude or phase in a lens pupil, on the other hand, is generally known as an apodisation or an optical filtering. Further, the double diffraction method is known as a method for restoring the reduced contrast of an image. These methods are discussed in, for example, Tsujiuchi, Progress in Optics, Vol. 2, pp. 133-152 (1983), North-Holland Publishing Co.